1. Field of the Invention
The present invention relates to a method of changing the code of a radio communication system, and more particularly to such a method which achieves the change of the code of a radio communication system through a gap between channels.
2. Description of the Prior Art
In a wireless telephone, a particular code is set for communication between the base and the hand set to prevent others from using the line unlawfully. However, because the code of a wireless telephone can easily be decoded by conventional techniques, the code must be regularly changed. Theexposure of the code of a wireless telephone to the public invites good-for-nothing persons to use the line without payment, and the wireless telephone owner will receive a high amount of debit note from the telephone (exchange) company. In order to protect a wireless telephone from being used unlawfully, the set code of the wireless telephone must be regularly changed. Even if the set code of the telephone line is decoded by an intruder, the intruder can only use the line unlawfully for a short period of time if the wireless telephone owner frequently changes the set code. Conventionally, there are two methods used for changing the code of a wireless telephone. One is achieved by means of charging terminals, i.e., the code is changed by means of the battery charger when the hand set is put back and attached to the base. It is necessary to attach switch means 12;15 to the base and the hand set when using charging terminals 11 and battery charger 11. When charging, the switch means 12;15 are respectively switched to the charging circuit 13 and the battery 16. When changing the code, the switch means 12;15 are respectively switched to the microcontrollers 5;1. However, because the signal for charge charging and the signal for code changing are different, it is complicated to design the switch means 12;15, and a software must be provided subject to a communication protocol for support. Therefore, this method is not economic. The other method is achieved by means the hardware of the base and the hand set. This method is less expensive, however it interferes with the communication signal in the channel under use when changing the code. In a regular 25-channel 46/48 MHz wireless telephone, the changing of the set code is achieved through the transmitting frequency of 43.72 MHz.about.46.97 MHz at the base and the transmitting frequency of 48.76 MHz.about.49.99 MHz at the hand set. However, because the 25 channels of the wireless telephone are of regular communication channels, using one of the 25 channels to change the set code may: (1) interfere with the communication of the subscriber's telephone, (2) cause a code change failure, and/or (3) cause a communication failure between the base and the hand set due to different codes.